GNOME Shell themes are only usable on GNOME 3, since GNOME Shell is a part of GNOME 3.

However, if the theme package also includes a GTK 2 theme and a Metacity theme (that's compatible with GNOME 2's version of Metacity and not just Mutter) you could install it and be able to use those to alter the appearance of your desktop environment (the GTK 3 and GNOME Shell themes will just be ignored). Do note, though, that most (if not all) GNOME 3 theme packages do not come with the required file for GNOME 2's appearance settings dialog for the theme selection pane (I have yet to see one that does come with the file), so you'd have to set it up manually.

As far as adding themes and icon sets to Gnome 3, I have found that many of the themes and icon sets from earlier Gnome versions will work in Gnome 3.

One of the first things I tried on the Gnome shell desktop with MGSE was sym-linking my collections of themes and icon sets to /usr/share/themes and /usr/share/icons. I found that about 75% of the hundred or so themes and icon sets I have acquired over time will work in Gnome 3.

I can't see any reason the icon themes shouldn't work, though some older ones no longer being actively worked on may not have the symbolic icons for the GNOME Shell (though in those cases I believe the Shell falls back to the full-colour icons anyway, so it shouldn't be a problem). The GTK2 themes should work fine as well for GTK2 applications, though if they don't also have GTK3 themes associated with them the GTK3 applications will probably default to the built-in, Win95-esque theme.

You must have a lot of themes that are still being actively developed and have had GTK3 themes added to them, I know the theme I used to use before GNOME 3 (Shiki-Colors) doesn't have a GTK3 theme and, while GTK2 applications were themed without issues, GTK3 applications were not and defaulted to the ugly built-in theme.

As I mentioned, not everything works in Gnome 3. I do have quite a few newer themes and icon sets since I only started saving them in my Data partition about a year ago and, on first look, it does seem that the newer items are the ones that work best.

karashata wrote:GNOME Shell themes are only usable on GNOME 3, since GNOME Shell is a part of GNOME 3.

It also seems not common knowledge in here that Gnome 3.0 extensions/shell themes usually don't work in current Gnome 3.2

Yeah, heaven forbid that 3.2 implies 3.0 backward compatibility. (sarcasm) This whole theme scheme is broken, IMO. Nobody can list a clear, full, set of instructions as to what packages are needed, what desktop mode (gnome, gnome-classic, etc), what themes to apply.